Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755351AbZF2IQe (ORCPT ); Mon, 29 Jun 2009 04:16:34 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751763AbZF2IQT (ORCPT ); Mon, 29 Jun 2009 04:16:19 -0400 Received: from mx3.mail.elte.hu ([157.181.1.138]:47526 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751019AbZF2IQS (ORCPT ); Mon, 29 Jun 2009 04:16:18 -0400 Date: Mon, 29 Jun 2009 10:16:15 +0200 From: Ingo Molnar To: Luming Yu Cc: LKML , suresh.b.siddha@intel.com, venkatesh.pallipadi@intel.com, Thomas Gleixner , "H. Peter Anvin" Subject: Re: [RFC patch] Use IPI_shortcut for lapic timer broadcast Message-ID: <20090629081615.GB571@elte.hu> References: <3877989d0906282347i311eb14bp80a7c80878219c31@mail.gmail.com> <20090629072047.GB20225@elte.hu> <3877989d0906290104q11a767b6qcc1acc4c2d1feba6@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3877989d0906290104q11a767b6qcc1acc4c2d1feba6@mail.gmail.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.5 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1356 Lines: 34 * Luming Yu wrote: > On Mon, Jun 29, 2009 at 3:20 PM, Ingo Molnar wrote: > > > > * Luming Yu wrote: > > > >> Hello, > >> > >> We need to use IPI shortcut to send lapic timer broadcast > >> to avoid the latency of sending IPI one bye one on systems with many > >> logical processors when NO_HZ is disabled. > >> Without this patch,I have seen upstream kernel with RHEL 5 kernel > >> config boot hang . > > > > hm, that might be a valid optimization - but why does the lack of > > this optimization result in a hang? > > It is hang caused by kernel code for work around lapic-timer-stop > issue. With HZ=1000, and a lot of cpus (eg. 64 logical cpus), cpu > 0 will be busy working on send TIMER IPI instead of making > progress in boot (right after deep-C-state has been used). that's a bit weird. With HZ=1000 we have 1000 usecs between each timer tick. Assuming a CPU sends to a lot of CPUs (64 logical CPUs) that means that each IPI takes more than ~15 microseconds to process. On what hardware/platform can this happen realistically? Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/